The present invention relates to a fuel fired burner, and particularly a gas fired burner, which preferably is of the fully premixed type ie. one in which the gas is mixed with all the combustion air in the mixing chamber before the gas is combusted.
Fully premixed burners have certain advantages over partially aerated type burners (ie. burners of the type in which only some of the combustion air is mixed with the gas before the gas is admitted to the combustion chamber.)
Firstly, the flames emitted from a fully premixed burner are shorter than those emitted from a partially aerated burner.
Secondly, it is possible to operate a fully premixed burner with a lower excess air level than is necessary with a partially aerated burner at the same gas input rate.
Thirdly, fully premixed burners can be arranged to fire the flame downwardly whereas partially aerated burners are generally unsuitable for such operation.
Taken together these characteristics means that fully premixed burners are ideal for use in boilers of the condensing type, since:
(i) the first and second characteristics enable the space required for mixing air and gas, and for combustion, to be minimised, thereby leaving a maximum of space for the heat exchanger within a given overall size of appliance casing. (Condensate type heat exchangers are inherently larger than non-condensing type heat exchangers of equivalent heat output).
(ii) the third characteristic permits the use of a simple system for condensate collection.
Both fully premixed and partially aerated burners can be operated over a range of gas flow rates ie. burner heat outputs. This range is defined as the turndown ratio which comprises the ratio of the maximum/minimum flow rates available for combustion. This ability is a great advantage in certain type of applications, for example, where the burner is to be used in a central heating boiler in order to maximise efficiency and to increase reliability of operation.
In partially aerated burners this ratio lies between 10 and 15:1 but in the types of fully premixed burner currently available for small gas appliances it is limited to no more than 2:1. This is because when the current types of premixed burner are operated at ratios of greater than 2:1 certain undesirable effects begin to appear.
Firstly, the phenomenon of "flame lift" begins to occur. In this case, the flame partially or completely detaches itself from the burner outlet ports and becomes very unstable, giving rise to poor combustion and/or burner shut down by the safety device which monitors the flame.
Secondly, the surfaces surrounding the ports may overheat and can become damaged, or can cause the flame to propagate back through the ports into the burner with disastrous results.
Thirdly, there is a tendency for the burner noise to rise to unacceptably high levels.